A variety of methods have been proposed for the remediation of soil containing organic contaminants. Many of the proposed methods involve removal and subsequent incineration of soil with the attendant difficulties of treatment and/or disposal of off-gases. A major detriment to such processes, however, is the cost of excavating and transporting the soil which can result in a total cost approaching several hundred U.S. dollars per ton of soil.
To avoid at least a portion of these costs, several types of in-situ soil heating processes have been proposed including steam or hot air heating of the soil through an auger system or through stationary pipes, radio-frequency, vitrification of the soil by electrode heating, or electrical heating of the soil by means of a surface heater.
An auger system for injecting steam or hot air has been practiced commercially. This method has limited use though, primarily in the decontamination of soil containing small areas of deep contamination such as localized spills or leakages at service stations.
A process for steam injection through stationary pipes has also been practiced commercially. This process which uses steam to heat the soil is limited by the amount of condensate that can be tolerated in the soil without causing contaminants to be leached to the ground water, or at least without clogging the soil pore volume with water which inhibits or restricts vapor or air flow. Also soil temperature is limited so it is practical only for the more volatile chemicals.
Brouns et al, U.S. Pat. No. 4,376,598, disclose a vitrification process in which the soil is heated to approximately 1500.degree. C. At or about this temperature, the soil forms a glass-like mass which traps the contaminants therein. This process destroys organic contaminants. However, for inorganic contaminants, it is, in reality, a stabilization process rather than a decontamination process since the soil undergoing treatment has lost its physical and chemical identity.
Bridges et al, U.S. Pat. No. 4,670,634, disclose an in-situ thermal process wherein the thermal energy is supplied by radio-frequency antennas for heating. The radio-frequency antennas may be placed on the ground or in the ground. This process is particularly applicable to water-containing soils where the steam generated in the soil serves to strip the organic contaminants from the soil. A somewhat related process is disclosed in Assignee's U.S. Pat. No. 4,984,594 wherein the thermal energy is supplied by a relatively flat electrical heater deployed at the surface of the soil. In this later process, a vacuum is applied at the surface of the soil to remove vapors generated within the soil.
U.S. Patent No. 4,842,448, issued to Koerner et al discloses a method and apparatus for in-situ removal of contaminants from soil comprising a barrier having a permeable inner layer and an impermeable outer layer overlying the contaminated soil and a vacuum system for reducing pressure under the barrier and withdrawing contaminants from the contaminated soil.
In Assignee's U.S. Patent No. 5,076,727, moist warm air from a vapor treatment system is injected into wells which are screened (perforated) only at the contaminated depth forcing air and vapor flow only through the contaminated region. Between the injection wells is an extraction well which is also screened only at the contaminated depth. A vacuum is drawn on the extraction well through the contaminated soil, thereby entraining contaminants. A microwave/radio frequency (MW/RF) heating system heats the earth's surface and the contaminated soil. By screening the wells only through the contaminated zone and maintaining the contaminated soil zone in a moist state, the entire energy system is focussed on the contaminated region.
U.S. Pat. No. 5,011,329 to Nelson et al discloses an in-situ decontamination method and apparatus for injecting a hot gas into boreholes formed in a contaminated soil area to vaporize the soil moisture and contaminants, and for collecting the vaporized contaminants at the surface of the soil. A burner heats pressurized gases and mixes the same with combustion gases for injecting into the contaminated zone. Controlled heating of the injection gas is effective to sequentially remove different types of contaminants, as well as to provide in-situ oxidation of other contaminants, while minimizing recondensation of the soil vapors.
In U.S. Pat. No. 4,593,760 and U.S. Pat. No. 4,660,639 to Visser et al, volatile contaminants are removed from the vadose zone of contaminated ground by pumping volatilized contaminants from the vadose zone using one or more vacuum extraction wells at ambient temperature.
It has now been found that a more effective distribution of heat and therefore better decontamination of soils containing volatile or semi-volatile contaminants may be achieved through the use of sheet metal pilings containing electrical heaters. In addition, such sheet metal pilings are particularly effective for treating areas having "tight" soil, since such soils do not typically conduct heat in an effective manner, and may be utilized also be particularly effective when used in particular configurations to treat specific sites and prevent the further spread of contamination.